Combustion stabilizer



June 6, 1961 YAO TZU Ll 2,986,883

COMBUSTION STABILIZER Filed Dec. 19, 1955 5 Sheets-Sheet 2 50 In yen 0r Yao Tzu Li By his AZZrneys June 6, 1961 YAO zu Ll 2,986,883

COMBUSTION STABILIZER Filed Dec. 19, 1955 5 Sheets-Sheet 3 oo (O N g I Q) k b @fi} g Q, $S EL u n: i l i E Inz/enfor Yao Tzu Li June 6, 1961 Filed Dec. 19, 1955 YAO TZU Ll COMBUSTION STABILIZER 5 Sheets-Sheet 4 Inventor Yoo Tzu Ll By his Afomeys I June 6, 1961 YAO 'rzu LI 2,986,883

COMBUSTION STABILIZER Filed Dec. 19, 1955 5 Sheets-Sheet 5 Yaq Tzu Li, Watertown, Mass.

United States Filed Dec. 19, 1955, Ser. No. 553,771 Claims. (Cl. 6039.69)

The present invention relates to apparatus for stabilizing combustion effects and more particularly to apparatus for preventing oscillations in the combustion rate in liquid fuel equipment as, for example, in liquid propellant rockets.

Liquid fluid burners, of which the liquid propellant rocket may be taken as an example, have a tendency toward oscillation because of the coupling between the fuel supply system and the combustion process. In rockets this occurs as low frequency oscillations, commonly known as chugging. The rate of fuel flow between the fuel source and the combustion chamber depends primarily on the pressure difference between the source and the chamber. On the other hand, the gaseous combustion products, because of their great volumetric expansion must be expelled through the nozzle with a high velocity and this requires the building up of a considerable pressure in the combustion chamber itself. Therefore, the rate of gas generation is a function of the fuel -flow rate, the chamber pressure is a function of the rate of gas generation, and the fuel flow rate is a function of the chamber pressure. These constitute the characteristics of a feedback or closed cycle system. For purposes of theoretical treatment such a system may be considered in the same manner as any feedback system, except that an analysis of the complete rocket system is made very complicated by reason of non-linear effects and because of insufiicient knowledge of many of the factors involved.

The low frequency oscillations, or chugging, appearing in the rocket have been recognized as a serious disadvantage, not only because of noise, but because of attendant inefficiency and uncertainty in operation. The oscillations are usually in the neighborhood of a few hundred cycles per second. Various schemes have been proposed in an effort to eliminate the chugging. One such arrangement, based on a theoretical analysis of the entire system, involves the use of a compensating network involving a pressure pick-up in the combustion chamber together with a suitable servo to control the fuel pressure. As a practical matter, such arrangements have not been satisfactory because present day servos cannot be made to operate at a sufficiently high frequency to be effective at the chugging frequency. Furthermore the complication of such apparatus introduces serious problems of weight, cost and reliability.

The simplest arrangement for eliminating chugging is to introduce additional restriction into the fuel line, thereby increasing the damping in the system. If the damp ing is sufficiently great the chugging can be eliminated or reduced, but this requires a high supply pressure. Although this method is widely used in test-stand operation, it is not satisfactory for airborne devices because of the excessive weight and decreased reliability. The effectiveness of the damping method does indicate, however, that it is possible to eliminate chugging by operations acting on the fuel supply part of the system alone, without the necessity for use of elaborate closed-loop compensating mechanisms.

It is therefore the object of the present invention to provide a simple and effective means of stabilizing combustion by elimination of oscillations, especially useful in liquid fuel rockets.

To this end and with other objects in view as will hereatent O inafter appear, the principal features of the invention comprise the introduction of a stabilizer or compensator into the fuel supply line, and responsive to the oscillations in the fuel supply, but without introduction of appreciable resistance to the steady state flow. My improved stabilizer may take either of two forms as will hereinafter be described, one of which responds to the rate of fluid flow and the other to the acceleration of the fluid.

Other features of the invention consist of certain novel features of construction, combinations and arrangements of parts hereinafterdescribed and particularly defined 1n the claims.

In the accompanying drawings:

FIG. 1 is a diagram of a rocket system.

FIGS. 2 and 2a are block diagrams of the system illustrating the conditions by which oscillations occur;

FIGS. 3 and 4 are diagrams showing the general theory of the present invention;

FIG. 5 is a sectional elevation of one form of apparatus according to the present invention;

FIG. 6 is a diagram illustrating the performance of the apparatus of FIG. 5;

FIGS. 7 and 7a are sectional elevations of modified forms of the apparatus of FIG. 5;

FIG. 8 is a polar plot showing the operation of the apparatus of FIGS. 5 and 7;

FIG. 9 is a sectional elevation of a further modification of the apparatus of FIG. 5;

FIG. 10 is a sectional elevation of another, and in some respects preferable, embodiment of the invention; and

FIG. 11 is a diagram illustrating the operation of the apparatus'of FIG. 10.

Theory Before going into a description of apparatus accord ing to the present invention I shall first describe the general theory on which my invention depends. In FIG. 1 is shown a diagram of a typical liquid rocket. There is a liquid fuel supply source 10 maintained under a pressure indicated as P, a fuel supply line 12, a com bustion chamber 14, an injector 16 for injecting the fuel into the combustion chamber, and finally a nozzle 18. Also shown at 20 in the fuel line is a stabilizer, which, for purposes of general description, may be merely a damper, or one of stabilizers of this invention to be presently described in detail.

For purposes of theoretical analysis there is shown in FIG. 2 a block diagram illustrating the feedback connections in a standard rocket, that is, one not involving the use of the stabilizer 20. This diagram is similar to the standard analytic diagrams for feedback systems generally and illustrates the coupling arising from the fact that the fuel supply rate depends upon the pressure in the combustion chamber which, in turn, depends upon the fuel flow rate itself. The diagram illustrates that, while the supply pressure is P, the system pressure difference is represented by P and is, in fact, the pressure difference between the supply pressure P and the combustion chamber pressure P The subtraction of the chamber pressure from the supply pressure to obtain the pressure difference P is illustrated diagrammatically by the feedback link 22 and the mixer 23 as is standard in servo analyses. The link 22 therefore represents at least a part of the internal coupling of the system by which oscillations are produced. The fuel flow rate is a function of P and depends upon the performance functions of the several parts of the system. The block 24 represents the fuel supply system itself, namely, the pipe 12 and the injector 16, and it also involves the mass of the liquid in the supply line.

and nozzle as indicated by the block 28. As heretofore stated, the effect of the combustion chamber pressure is manifested through the diagrammatic feedback link 22.

The complete theoretical analysis of the quantities in- 'volved in the diagram of FIG. 2 is exceedingly difficult,

if not impossible. However, my invention is concerned only with the characteristics of the fuel supply system itself, as distiguished from the whole system, because as heretofore noted, my principal objective is to correct the chugging condition by operations only on the fuel supply and without the necessity for a complex servo control system. I have discovered in this way that a substantial simplication of the essential theory may be attained.

In FIG. 2a I show the internal characteristics of the fuel supply system alone as represented by the block 24 by itself. If the flow'were a steady flow only, a flow rate F would be obtained. However, because of the mass of the fluid existing in the pipe 12 and the fluctuations in fluid by reasons of the conditions heretofore mentioned, there is a pressure AP at chugging frequency and there is a variable flow rate. AF superposed on the steady state flow F The conditions are as if there were an internal feedback loop 30 connected from the outlet side of the injector 16 back to the beginning of the line 12. The

' characteristics of the feedback loop may be represented by the expression where D is the differential operator, L is the length of the supply pipe, p is the density of the fluid and g is the gravitational constant. In other words, the effect of this loop is to introduce a change of pressure AP dependent on the rate of change of fluid flow through the pipeline 12. If the flow were perfectly steady AF would be zero and the differential operator would operate only on the constant F and in that case AP would be zero. In the actual case the operation of D upon the flow rate results in an acceleration which is converted into pressure by the term In feedback analysis it is customary to obtain a performance function involving the frequency scale, using the quantity D as a Laplacian operator. The part-system of FIG. 2a is a first order system, in which the performance function (PF) is a simple expression, as follows:

The time constant is therefore a measure of the lag introduced into the system by reason of the fluctuations of the flow in the fuel-supply line.

In keeping with the usual analytical technique of diagramming characteristic functions, I show in FIG. 3 a polar plot of Equation 1 illustrating the performance function for different frequencies. This is in accordance with the usual technique of setting D=jw, where j is the imaginary operator, and w is frequency. The plot is 'm-ade non-dimensional by plotting amrather than 0:. If

there were no oscillations in the line the characteristic would be represented by the arrow lying along the axis of reals. However, the actual characteristic is represented by an arrow such as is shown at 32, indicating a phase lag at all frequencies other than zero. Since the system plotted in FIG. 3 is a first order system, it does not by itself indicate any oscillation, since a true first order system operating by itself is not oscillatory. However, it is known that the cause of oscillations in any feedback system is the accumulation of lags in the system as a whole (if there is suflicient gain in the system). It is also known from experience that a fuel supply system having the theoretical characteristics of FIG. 3 can result in chugging oscillations of the system as a whole. Such chugging will exist because of the phase lag and gain represented by the characteristic 32 contribute to the phase lags and gains of other parts of the system to satisfy the criteria for oscillation.

The present invention comprises the provision of means for modifying the characteristic of FIG. 3 in such a manner that chugging oscillations will not be maintained in the system as a whole. The invention comprehends two specific embodiments, one of which comprises the insertion of a sufiicient phase lead to prevent occurrence of sustained oscillations in the system. The second embodiment of the invention comprises the reduction of gain of the system so that oscillations cannot be sustained. The first embodiment involves a decrease in the time constant and corresponds to a shift of the operating point from a to b of FIG. 3, while the second embodiment may be constructed to give either a decrease of the time constant, or an increase thereof, involving a shift from a to c. In the latter case there is an increase in phase lag which by itself is undesirable, but which is more than balanced by the reduction in amplitude.

It is known that an increase of damping by the provision of a restricted orifice in the fuel line results in the elimination or reduction of the chugging effect. Such a restriction requires an increase of pressure for the steady state flow and is hence undesirable as a practical operating means. It is known, however, that the addition of such a restriction decreases the time constant of the system and therefore introduces a lead and also reduces the gain. As indicated in FIG. 4, the characteristic 34 represents the original characteristic similar to that shown in FIG. 3, while the curve 36'represents the characteristic as modified by increased damping. The characteristic of the added damping device'itself is indicated by the dash curve 38. If a rocket has been found to oscillate at a chugging frequency represented by the arrow 40, the modified performance function upon the introduction of an increased restriction is represented by the arrow 42 which terminates on the characteristic 36. The phase lead indicated by the angle A has been introduced by the restriction and the amplitude has also been reduced.

Rate-actuated stabilizer According to the first embodiment of the present invention (shown in principle in FIG. 5) there is provided a compensator to be introduced into the fuel line which produces a phase lead and a gain reduction at least as great as that of a damper. In other words, if the amount of restriction necessary to eliminate chugging is known, a device of the type shown in FIG. 5 may be employed which gives at least as good a modifying characteristic as the damperat or near the chugging frequencyJ It is important to note, however, that the device shown in FIG. doesnot materially impede-the steadystate flow and hence does not require the use -of equipment for providing exceptionally high pressure at the source. In

other words, the apparatus provides a frequency-dependent restriction factor so that sufiiiciently large damping can be obtained in the vicinity of the chugging frequency.

The pipe 12 leads into the body 44 of the compensator within which is mounted a hollow cylindrical valve ber 46 adapted for sliding motion within the body. E'I'he valve is centralized by means of opposed compression springs 48 and St), the former bearing against one end of the pipe 12 and the latter bearing against a boss 52 which is of a shape to form outlet passages 54 around it. The fluid from thepipe -12 passesthrough' the interior of the sliding valve 46 and then turns substantially at right angles to go through the passages 54 into the outlet 56 leading to the injector and'rocket motor. The valve 46 is formed at its entrance end with a smoothly curved converging surface 58 and is formed at its exit end with a smoothly curved surface 60. The interior of the valve therefore constitutes a nozzle which serves as a flow sensing element. A difference of pressure exists between the entrance and exit ends of the nozzle and such pressure depends upon the flow rate. Thus the valve tends to move into a position determined by the flow rate. An increase in flow rate will tend to"dirninish the nozzle exit passage 54 and thereby impose a restriction in the flow. Under'steady state conditions the nozzle assumes a position'partly closing the '-passageso that the valve can move one way or the other upon an increase or decrease of the fuel rate. The -'size of the passage under steady state conditions is such that 'no undue restriction occurs, but the resistance to new un'der dynamic conditions is greatly increased.

Under idealized conditions, if the mass of the piston 46 and the resistance to its movement by frictio'n or damping are neglected, it can be shown that the apparatus of FIG. '5 can be made to have a characteristic similar to the simple restriction characteristic 3801? *FIG.

4, except that a high pressure difference 'is not needed at the operating point. 'The restriction factor R- or a simple orifice is given by:

1 ..Fnr

where F and P represent the flow rate through the orifice and the pressure across it respectively. For the idealized stabilizer of FIG. '5, the corresponding restriction factor R is given'by:

'be made equivalent to R of (3) with only a moderate pressure drop P across the stabilizer. In other words, a stabilizer characteristic like 38 or better may be obtained without imposing excessive restriction to steady state flow.

The conditions are illustrated by the plot of FIG. 6. The curve 62 is atypical characteristic fora fuel supply line with a small restriction For a given flow F repre sented by the horizontal dash line 64 a pressure of only P is required. The dynamic characteristic is represented by the derivative of F with respect to P, namely, by the slope of the tangent as indicated at 66. With such a small restriction oscillations at some frequency in the system as a whole are likely to occur. The addition of a further restriction results in a characteristic 68 showing that for a given flow "rate F a greatly increased supply pressure P will be required. The slope of the tangent is much less, as 'is indicated at 70, show- FIG. 5, however, it is possible to obtain a static characteristic similar to the curve 62 and a dynamic characteristic similar to that of the curve 68. The static characteristic is given by the curve 72 which passes through the same static operating point as the curve 62 so that an average flow F is obtained with only a pressure P The dynamic sensitivity is given by theslope of the tangent 74 which corresponds approximately'with 70. The general shape of the characteristic 72 as shown in FIG. 6 can be'confirmed by'the fact that at low flows there is little or'no restriction due to the valve, whereas at high flows the valve moves toward a closed position.

The theoretical discussion concerning the'rate-actuat'ed stabilizer in FIG. 5 has neglected the mass of the piston and that of the fluid included within the piston, as well as the damping 'or friction against the motion 'of the piston. In taking account of these factors itwill be seen that the apparatus of 'FIG. 5 constitutes a second-order system. Stated in another way, since the system has mass, its dynamic characteristic is represented by 'a differential equation which must be at least of the second order. Such a system has a more complicated characteristic than a simple orifice, and in a theoretical treatment it is necessary to take account of the undamped natural frequency of the stabilizer itself.

In FIG. 8 I have repeated the characteristic 3813f a simple orifice together with the modified fuel line characteristic 36. Values of w'l' are indicated on thesecuives. At 62 I show a second order characteristic for a typical device shown in FIG. '5 and "at 64 the fuel line'characteris'tic as modified by the stabilizer characteristic 62.

The polar plot '64 is aplot of "the following performance function representing -therelatio'n between "pressure and ilow rate as a function of frequency:

(PF): s

-1+ 7rw+------ w 2 (5) where 1' is as defined before, his the -f'o'rcing frequency, (an is the undamped natural frequency of the stabilizer, and {is a damping coefficient. The factor K is ratio S Where 8,, is the sensitivity of the unstabili'zed injector system as previously defined, while S is the static sensitivity of the stabilizer. v

The modified fuel line system characteristic 64 depends to a considerable extent on the undamped natural frequency of the stabilizer. The particular curve 62 shown in FIG. 8 is for an undamped frequency o such that w r=4, and :0.5. On these assumptions it will be seen that for an actual frequency such that (OT-":1 the rate stabilizer provides more lead and more gain reduction than the simple orifice 36, but for higher frequencies, such as (oi-=3, the rate stabilizer shows an increase in gain over the simple orifice, although it still shows an advantage in phase lead. If the chugging, frequency is in the neighborhood of spect to flow rate. In order to obtain a high valve port sensitivity the construction shown in FIG. 9 is preferred. In this construction instead of merely using the piston 60 to close off the port 54, a sleeve valve 66 is provided having a number of openings 68. The valve cooperates ing that oscillations may be "damped out at the expense 75 with lands 70 which areof a'width comparable to that of .scale in FIG. 7a.

and rear.

' move to the left and therefore widenthe inlet port.

outlet port.

'1 each of'the openings 68. It will be'seen that by the provision of a suflicient number of openings, the total valve width may be made substantially the same as that of a single passage, but the valve area may be completely closed oif with a relatively small movement of the piston. By this means it is possible to obtain a high undamped natural frequency, and still have the stabilizer operate With a sufficient sensitivity to provide an overall characteristic for the system which is at least as stable as that obtainable by a simple orifice type restriction, but without the necessity for providing a high supply pressure.

A modified, and in some respects improved, form of rate stabilizer is shown in FIG. 7. In this construction the piston valve is made of two concentric sleeves and the object isto provide a frequency sensitive device which will make the stabilizer effective near the chugging frequency and substantially ineffective under steady state conditions, whereby no substantial restrictions are imposed against a steady flow. To this end there is an outer nozzle valve 72 and an inner sleeve 74 adapted to slide longitudinally relative to one another. The shape of the combined parts 72 and 74 is similar to that of the valve 46 of FIG. 5. The outer sleeve 72 is centralized by two springs 76 and'78 while the inner sleeve is centralized by two springs 80 and 82. The sleeves are constructed to provide a cavity 84 between them. One or more passages 86 between the sleeves provide for flow of fluid into or out of the cavity. A simple reed valve 88 overlies the inner end of each passage 86, as shown on an enlarged Under steady conditions the outer sleeve 72 remains stationary, or nearly so, because the principal force due to the flow is exerted on the inner sleeve 74 and there is little or no shearing force between the two sleeves. However, when the sleeve 74 oscillates because of fluctuating flow in the system, fluid is pumped into the cavity 84 past the direction sensitive valve so that a force is transmitted from the inner sleeve to the outer sleeve to tend to close the valve.

Acceleration-actuated stabilizer The apparatus shown in FIG. 10 also provides stabilization by actions occurring only in the fuel supply line, but on a somewhat diflferent principle from that of FIGS. and 7. In the line is mounted a body 90 having an internal stationary memb'er'92 suitably streamlined at front Within the member 92 is a sliding piston 94 centralized by springs 96 and 98. The piston is reduced in diameter to provide a flow passage in the form of a cavity 100 longitudinally of its outer surface. An inlet port 102 and an exit port 104 in the member 92 communicate with the passage 100 in the piston 94. The inlet and outlet ports 102 and 104 are arranged so that the flow is guided to enter and leave the cavity 100 in directions perpendicular to the axial motion of the piston. Under these conditions the inlet and outlet flow stream does not exert any force on the piston in the axial direction. The axial force acting on the piston is entirely due to the change of momentum of the fluid in the cavity 100.

In the arrangement shown in FIG. the piston is shown in its neutral position, the inlet port being partially covered and the outlet port being uncovered. Whenever there is an increase in the rate of flow the piston li vll lll e resulting effect is to decrease the pressure drop through the stabilizer whenever the flow rate increases; that is, whenever there is an increase in the fioW rate.

An alternative construction is to centralize the piston 94 in its normal position such that the inlet port is completely uncovered while the outlet port is partly covered. This condition may be represented in the drawing simply by considering that the flow directions in FIG. 10 are reversed, whereby 104 becomes the inlet port and 102 the In such a case the opening of the port would be narrowed on an increasee of flow rate.

--Thus the device 'of'FIG. 10 provides an acceleration term in the performance function, which term is positive for the flow direction actually shown in FIG. 10 and negative if the apparatus is set up to produce a flow in the reverse direction. Either the positive or negative term may be used, as will be described, although the actual quantities that are involved differ somewhat in the two cases.

In either case the acceleration term is coupled with a flow-rate term because of the fact that a certain pressure difference is necessary to force the fluid through the ports 102 and 104 even when there is no acceleration of the fluid. However, if the sensitivity to acceleration is high, as it may readily be made, the flow-rate term may be neglected.

An analysis similar to that of Equation 5 may be carried out, and it is found that the performance function for the entire fuel supply system is as follows:

where the quantities are as defined before, except that K is a quantity involving the sensitivity of the apparatus of FIG. 10. The plus-or-minus sign is used to distinguish the positive and negative acceleration terms, depending on whether the pressure drop increases or decreases upon an acceleration of the fluid.

Atypical plot of the conditions is given in FIG. 11. This is for the case of the positive acceleration term, as represented by the flow direction indicated in FIG. 10, and is for the condition w T=1 and =0.3. The plot for the'system modified by the acceleration-actuated stabilizer of the present invention is at 110, and for comparison I show a plot 112, similar to FIG. 4, for a system modified .by-a simple restriction. From this comparison it will be seen that if the natural frequency ta is chosen to be about equal to or greater than the chugging frequency, the acceleration actuated stabilizer will produce more phase lead and less gain than the simple restriction.

It will be noted that the rate-actuated stabilizer plotted in FIG. 8 showed desired stabilizing characteristics for a natural frequency several times the chugging frequency, while the acceleration-actuated device gives satisfactory stabilization if designed for a natural frequency approximately equal to the chugging frequency. The acceleration-actuated stabilizer is preferred over the rate-actuated stabilizer because of its lower natural frequency.

If the negative term of (.6.) is used, as represented by a flow direction reversed from that of FIG. 10, the system a polar plot would show. That the device introduces additional phase lag over a wide range of frequency, ,as compared with a simple restriction. .While increased lag is ,in itself objectionable, the amplitude is greatly reduced, and hence chugging is eliminated by the fact that the overall gain around the system is insufiicient to maintain oscillations.

In the acceleration-actuated device, the valve-sensitivity should be kept high, as in the rate-actuated device. Therefore, a valve arrangement like that of FIG. 9 is preferred; and further sensitivity may be obtained with multiplying linkages.

Having thus described the invention, I claim:

1. In a liquid fuel system having a fuel source, a combustion chamber and afuel supply line, said system being subject to oscillations because of the effect of the combustion chamber pressure on the fuel supply, the combination of a compensating device having a valve member to control a variable-area passageway in the supply line, said valve member having a normally open passage to introduce no substantial restriction to steady flow, centralizing springs for the valve member to hold the valve member in a position to determine a steady flow 9 rate under non-oscillating conditions, and means operated by variations in flow for reciprocating the valve to vary the passageway at a frequency to alter the dynamic characteristics of the fluid flow to suppress said oscillations.

2. In a liquid fuel system having a fuel source, a combustion chamber and a fuel supply line, said system being subject to oscillations because of the effect of the combustion chamber pressure on the fuel supply, the combination of a compensating device having a valve member to control a variable-area passageway in the supply line, said valve member having a sliding piston with a passage therethrough leading to the variable area passageway, centralizing means for the piston to hold the piston in a position to determine a steady flow rate under nonoscillating conditions, and means for turning the path of fluid at an angle upon exit from the piston, whereby the piston reciprocates at a frequency dependent on the variations in flow to suppress said oscillations.

3. Apparatus according to claim 2, in which the piston comprises two parts arranged to move with respect to each other, said parts having a cavity between them, and a one-way valve operable under oscillating conditions to cause fluid to be pumped into the cavity.

4. In a liquid fuel system having a fuel source, a combustion chamber and a fuel supply line, said system being subject to oscillations because of the effect of the com bustion chamber pressure on the fuel supply, the combination of a compensating device having a valve member in the supply line, said valve member having a normally open passage to introduce no substantial restriction to steady flow, said valve member to control a variable-area passageway in the supply line, said valve member having a sliding piston with a longitudinal fluid passage, means for conveying fluid into and out of said passage, the valve member having valve ports arranged to be variably covered by the piston and constituting the variable area passageway, and a centralizing mounting for the piston to hold it in a position to determine a steady flow rate under non-oscillating conditions, and means for turning the path of fluid at an angle upon exit from the piston whereby the piston is caused to oscillate in accordance with accelerations of the fluid under fluctuating conditions to vary said ports and thereby suppress the fluctuations in the fluid flow.

5. In a liquid fuel system having a fuel source, a combustion chamber and a fuel supply line, said system being subject to oscillations because of the effect of the combustion chamber pressure on the fuel supply, the combination of a compensating device having a valve member to control a variable-area passageway in the supply line, said valve member having two relatively sliding pistons having a passage therethrough, and separate sets of centralizing springs for the pistons to determine a steady flow rate, means operated by variations in flow for reciprocating the pistons to vary the flow rate at a frequency to suppress said oscillations, the pistons having a cavity between them, and a one-way valve to control passage of fluid into and out of the cavity under conditions of relative movement of the sliding members.

References Cited in the file of this patent UNITED STATES PATENTS 2,295,044 McCarty Sept. 8, 1942 2,389,887 Baxter Nov. 27, 1945 2,681,695 Bills June 22, 1954 2,722,180 McIlvaine Nov. 1, 1955 2,729,235 Stevenson Jan. 3, 1956 FOREIGN PATENTS 288,716 Germany Nov. 13, 1915 

